The use of the internal combustion engine is abundant. Since internal combustion engines burn fossil fuels within the cylinders of the engines, gases are emitted that are potentially both harmful to human health and harmful to the environment. Over the past few decades, internal combustion engine usage has risen dramatically with the attendant increase of environmental pollutants.
The main gases of concern emitted from the exhaust system of a vehicle are i) Hydrocarbons (HC) which comprise unburned or partially burned fuel and which, in addition to being toxic, are a significant contributor to smog in urban areas; Nitrogen Oxides (NOx) which are created when nitrogen in the engine air flow reacts with oxygen in the high temperature-pressure conditions inside the combustion cylinders and which are a factor in both smog and acid rain; Carbon Monoxide (CO) which is a product of incomplete combustion and which is also toxic to humans; Carbon Dioxide (CO2) which is a by-product of the combustion process and is considered one of the most significant green house gases; and Particulate Matter (PM), also referred to as soot.
The types of internal combustion engine that are in most widespread use in on-road and off-road applications are the spark ignition engine (gasoline fuelled) and the compression-ignition (diesel) engine.
In a spark ignition engine a throttle valve controls the rate of air supplied to the engine in response to a power demand by the vehicle operator such that a fuel supply system supplies an amount of fuel based on the air supply rate to obtain a desired air/fuel ratio. To reduce the exhaust emissions from such an engine it is known to use a catalytic converter having the functionality of a reduction catalyst to reduce NO2 to nitrogen and oxygen and an oxidation catalyst to oxidize CO to CO2 and HC to water and CO2. For optimum operation of the catalytic converter, the spark ignition engine is operated under stoichiometric operating conditions in which the amount of oxygen supplied to the cylinders of the engine is the exact amount required to completely combust the amount of fuel supplied. In the case of gasoline the stoichiometric air/fuel ratio around is 14.7:1, although the exact value depends on the fuel composition.
In contrast to spark ignition engines, diesel engines are operated under diesel combustion conditions in which the air/fuel ratio is much leaner than that of the spark ignition engine, typically between 20:1 and 40:1.
In a diesel engine, air is introduced into the cylinder head via an inlet valve and is compressed during an compression stroke of the piston which compresses and heats the induced air. Fuel is injected directly into the cylinder at close to top dead centre piston position and is ignited by the heated air which causes combustion and a corresponding rapid expansion of the compressed air/fuel mixture that drives the piston on a power stroke. The air inducted into the cylinder is not throttled as it is in a gasoline engine and as a result a diesel engine naturally runs at a much higher air/fuel ratio.
The diesel engine has many benefits. For example, as a result of its leaner combustion regime and higher compression ratio, the diesel engine has increased thermal efficiency over gasoline engine which translates into a greater torque at comparatively low engine speeds and a greater fuel economy.
One of the characteristic emissions of the diesel engine is the generation of particulate matter or soot, in addition to the emission of hydrocarbons, carbon dioxide and the like. To combat the emissions of diesel engines, it is known to employ various devices to clean the exhaust gases. For example, it is known to place a diesel particulate filter (DPF) in the exhaust system to reduce approximately 80% to 100% of particulate matter entrained in the exhaust gas flow.
It is also known to use a diesel oxidation catalyst (DOC) which uses excess oxygen in the exhaust gas flow to oxidize CO to CO2 and HC to water and CO2. However, such a device is unable to reduce the NOx present in the gas flow significantly.
A major challenge facing the future of the diesel engine is the reduction of NOx output in a cost-efficient manner.
In general, two main approaches are used in an effort to reduce NOx emissions. Firstly diesel engines are run with extremely high exhaust gas recirculation (EGR) rates in an effort to lower the combustion temperature of the cylinders thus reducing NOx generation. Although a high EGR rate is effective at engine idle and relatively low engine load conditions, it is difficult to obtain high EGR rates at medium to high load conditions, for example during acceleration phases of a vehicle since a relatively high amount of fresh air is required for combustion. Secondly, it is known to use devices generally referred to as NOx absorbing devices (“NOx traps” or “NOx absorbers”) and selective catalytic reduction (“SCR”) devices, both technologies being known to the person skilled in the field of diesel engine technology.
However, SCR devices tend to be expensive and need a minimum temperature to operate efficiently which requires regimes to heat up the device: typically this is via late injection of fuel which leads to oxidzation of fuel in the SCR device in order to raise its temperature. SCR devices are generally undesirable due to the complexity of the apparatus itself, the associated control system, and the fact that it impacts adversely on the engine fuel consumption. As for NOx absorbing devices, they require periodic regeneration in order to maintain their effectiveness which, again, impacts adversely the engine fuel consumption. Moreover, the presence of sulphur in diesel fuel can degrade the performance of NOx absorbing devices which limits their use to geographic areas in which the fuel supply is of acceptable quality.
As can be appreciated, the provision of several of such devices in an effort to combat the varied emission from the diesel engines results in an excessively costly system that, in general, lacks widespread viability for commercial vehicles and smaller vehicles for the private motorist.
It is an object of the invention to provide a diesel engine system that avoids or at least mitigates some of the problems explained above.